Electrical amplifier circuit



Feb. 17, 1931. L. M. HULL ELECTRICAL AMPLIFIER CIRCUIT Filed Dec 15, 1928 5 Sheets-Sheet l gwwnioi:

Feb. 17, 1931. HULL 1,793,367

ELECTRICAL AMPLIFIER CIRCUIT Filed Dec. is. 1928 5 Sheets-Sheet 2 .Mefers )1 Feb. 17, 1931. 1.. M. HULL 1,793,367

ELECTRICAL AMPLIFIER CIRCUIT I Filed Dec. 15, 1928 5 Sheets-Sheet 3' Mefers 2 gwuemtoi:

Feb. 17, 1931. L. M. HULL ELECTRICAL AMPLIFIER CIRCUIT Filed Dec. 15, 1928 5 Sheets-Sheet 4 Ba/ance Capaci/Y Fequ/hed far/4 iir grfid.

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gwwln l ok Feb. 17, 1931. L. M. HULL ELECTRICAL AMPLIFIER CIRCUIT Filed Dec. 15, 1928 5 Sheets-Sheet 5 Patented Feb. 17, 1931 UNITED STATES LEWIS M. HULL, or nom nally LA nsjinw amen; Assrenou To RADIO F EQU NCY LABORATORIES, noonronn'rnn, or BooN oN, NEWJERSEY, A" CORPORATION "on NEW JERSEY,

mama; murmrrnn cino'uir- I Appl ieation fil ed ecember '15, 1928. Serial No. 326,345. v

This invention relates .to electrical amplifier circuits of the't-ype emplo'ying electron or vacuum, tubes including an emitting cathode, an anode or plate, and one or more additional elementsusually having the form of grids, one ofiwh'ieh ordinarily'servesas thecontrol element's Such'tubes are now commonly known as audions,'t-riodes tetrodes and so forth. Theinvention re-' lates especially to a balanced, amplifier c1r-, cult of the foregoing type,;fand provides. means for reducing},eliminatmg, or control ling retroaction orregenera-tion from the out put circuit of'such an amplifier stage to the input circuit thereof. This retroaction or regeneration may be caused by coupling due to capacitiesexisting between the elements of the tube andth e connections associated therewith, and especially by coupling due to capacity between the-anode orplate and control element or grid ofthe tube. The inven-' tion relates also. to a'balanc'ed amplier circuitof the foregoing type in which such retroaction or regeneration may be controlled by means of a deliberatepartial unbalance of the circuit, and particularly to :such a circuit in which the amount ordegree ofsuch unbalance may increase-with increasing wave length at a predetermined rate over a part or all of a given range of wavelength,

Referring now to the drawingsvFigure 1 shows a portion of a! multistage electrical amplifier including audions of the three-electrode type and arranged according to one embodiment of my invention. Figure 2 shows a similar amplier circuit arranged-according to another embodiment of myinvention; Figures 3a, 3b, and 3d are curves illustrative of the-operationiof thecircuit shown in Figure 1. Figure 4 shows similar curves illustrative of the circuitf shown in Figure 2. p 1" 1'1.

Inthe arrangement shown in Figure 1, T is a'three-element audiontube provided with a plate P, a grid G, and a' filamen t F. The latter may suitably be of direct current, A. C., or heater type, and is here shown as an A. C. filament supplied with heatingcurrentgfrom a houselightin source of alternating current through a suitable transformen; -A shunt resistance Rmay be connected across the filament, each side of which may also be by passed to groundby means ofby-pass condensers C as shown, in the well-known manner. Aigrid-biasing resistanceR' may also beconne ct'ed between a'suitable pointin the filament system and ground, as shown. 7 The input circuitfor the amplifier stage including the tube T may assume any desired form, and is here shown as including the tuned circult Ila-C of whichL constitutes the s'ec-* on'dary' of an input'transformer L L As representative of the impedanceof a preced-- ingamplifier stage, as is the case in'a multistage arrangement, :is shown the resistance R equal to say l0,000 ohms,' connected inseries' with the input transformer primary. 'The 'plate coil of tube "T or output transformer primary, L is connected between the anode P and the cathode F, preferably in series with a suitableB-battery or other source of plate potentiah which may be by passed in a well-known mannerby the condenser C. Coupled to the' output trans former priinary L with mutual inductance M is the output transformer secondary L These primary and secondary 0011s have negative coupling, that'is, one coil is wound or connected in reverse sense as regards the other, so that if an alternating current is assumed to how through the series resonant circuit of whichL form's apart, the voltage drop across Lg due to thiscurrent willbe substantially opposite i n phase to the voltage drop across the primary coil L induced therein through the mutual inductance M by ,thecurrent flowing in L Thile' it is possible to employ substantially unity coupling between the coils L and L it is ordinarily not necessary to do so, and'I usually prefer to employ less than unity coupling be- 7 tween them, thereby avoiding the high" losses which are apt to take place inphysical constructions employing unity coupling be tween reversed transformer coils. 'In series with the secondary L is connected a compose] ite impedanceincluding a balance coil L and an unbalancing' capacity, C' The coil L and capacity 0' which constitute this come posits impedance may-"be arranged eitherliin pedanceL series, asshown in Figure 1, or in parallel as shown 1n Figure 2, as will be hereafter described, and the capacity 0 3 may be constituted either by a separate condenser or, wholly or in part, by distributive capacity of coil L The secondary L is tuned by tuning either the'c oil L or the coil L and the" coupling between the balance coil and both the primary and the secondary is In any case negligible. I find that such coupling is ob tained, for all practical purposes, when the mutual inductance between thekbalance coil and the secondary is'negligible'in comparison with the self-inductance of these coils. Such negligible coupling may be provided by the use ofan electricaltransformer inwhich the coil L is mounted at right angles to theprimary and secondary'coils, as is described and claimed in the c'opending application of Frederick iPLDrake and William D. Loughlin, Serial No. 275,449, filed May 5,1928 A balance condenser C may beoconnected between the grid G and point X between the coils L2 and L although in certain cases the natural capacities otpthe circuit, between the elementsthereof, may serve as the partial or complete equivalent'of theicapacity G Since thecathode F is grounded, asis the low po:

tential side of the tuning condenser C 'at its junction with the composite impedance L ;'C a connection exists between these points; This is of advantage for example in multistage amplifiers employing a ganged tuning condenser, andin other cases.

An approximate balance condition for the circuit illustrated in Figure 1 may be expressed as follows:

g QM l m 3 n v wG'a when , v w L 0' l (2) where C is equal to the total effective gridplate capacity of the tube T w=the angular Velocity ofthe'current, and the other symbols designate the magnitude of the corresponding elements as indicated in the drawing.

As one-specific illustration of certain constants which I have found to produce s'atis factoryoperation in thecircuit of Figure 1, I may mention thefollow'ing:

Tube =UX 226 E; 1.5 volts' E0 (grid bia's)='6 volts EB =94vo1ts v (a) 20 turns #36 enamel copper wire'on 1 form, L s 1" winding space l i a 1 (b) 20 turns nichrome wire on 1' form, 1' wind-. ing space; L11 87turns #39 enamel copper wire on 1i form L3 --'50 turns #30 enamel copper wire (may form 7 0'3 .002 m. f.

{For the-circuit balanced at-a wavelength of 200 meters by means of balance capacity C Figures 3a and-3b show sample curves obtained by plotting the gain per stage E/E where E is the voltage across the output tuning condenser and E the'voltage across the input .turiin condenser, gainst wavelength in meters, for difl'erent values in microfa-rads of the capacity C and for the copper-wire and resistance primaries, respectively, and other circuit constants noted "above. Inthis as well as other modifications ofthe present invention, the unbalance may be controlle d and the shape of the gain characteristic altered, by means of the relative magnitudes of the elements causing seconda'ry'nnbalance, i: eL composite impedance L G= balance capacity-"C' and so forth, or' by means of the resistance in the primary circaitindading L With regard-to these curves it will be noted that the curve marked G =infinity indicates the operation of the circuit when the con denser C is omitted from: the composite imped-anc elh ah while the other curves show theimproved operation and amplification of the circuit for different values of G' -i The curve'd m'arke'd-'CV- ='.002 m. f., for a resistance primary shows, for example, a high gain characteristic for an amplifier stage: 1 v

Figure 3c shows -the various amounts of balance capacity Cgirequiredto produce an aural 'balance at a wavelength of 200 meters 'fofvariousvalues ofthe capacity G' in the composite impedance. Figure 3d shows the balance capacity C required to produce an aural "balance at'va'rious wavelengths for C' =.0O5 f. Taken together,'these curves show the progressive'unbalance of'this circan; obtained under the conditions indicated.

Figurefi illustrates aportion of a multistage electrical amplifier circuit which'is' sime ilar to that shown in; Figure 1, except that in this arrangement the composite impedance L ,C is arranged as a parallel insteadof a series circuit. The other connections correspond with those shown in Figure 1, although a portion of a preceding stage is here illustrated as includingthe'tube T,'rather than-merely the equivalent resistance R As an'ex'ample illustrative of suitable constantsforaciruit arrangement such as that of FigureQ, I may mention-the following:

, #36 enamel copper wire on l iorm, 1' In length of 200 meters, typical gain character- I istics are indicatedby' the curves in Figure 4,

network or which are, respectively, for a copper wire primary and a resistance wire primary. I Variations or these gain characteristics can, of course, be produced as explainediabove, either by adjustin the secondary unbalance of the by varyingthe resistance in the primary circuit.

I tion for the GlICLlltSllOWIl 1n expressed as follows:

. .An approximate theoretical alance condi- Figure2 may be I Q M12 O T L cuit is intended to operate, and where thesymbols have the meanings indicated in connection with equations 1 and-2 above.

The application of the present invention is not limited to audion tubes of the three-element type, and itmay be applied, when desired, to tubes of the so-called double-grid or shield grid, or other, type. In such case the control grid of the tube is ordinarily connected as is the grid G in Figures 1 and 2, al-

though suitable modifications of the circuit may 0]": course be made.

The present invention constitutes an improvement upon the invention described and claimed in U. S. Patent No. 1,672,811, and also, in the cases where resistance primaries are employed, may constitute an improvement upon the invention vdescribed and claimed in the copending application of Frederick H. Drake and Paul O. Farnham, Serial No. 275,189, filed May 4, 1928.

In electrical amplifier circuits of the balanced type the amplification or gain is apt to be higher at short wavelengths than at longer wavelengths. For many purposes it is desirable, however, to make the amplification or gain in the stage at the longer wavelengths on which the amplifier may be called upon to operate, higher than it would ordinarily be, and often at least as high as the gain at the shorter wavelengths of the wavelength I range of the amplifier. Through the use of thepresent invention, as will have appeared from the foregoing description, the amplification is increased at the longer wavelengths of the wavelength range of the amplifier by proportioning the elements of the network so as to give a controlled secondary unbalanceconducive toregeneration, and this regenerative unbalance may suitably be arranged'to increase with increasing wavelengths. This increase may also be made at a predetermined rate by proper proportioning of the circuit constants in accord with the explanations given above. I I

The specific circuit arrangements and examples of circuit'constant's' given herein are: I

only illustrativeof my invention, which is variations and'modifi'cations thereof.

Iclaim:

not limitedtheretm but which includes many I combination, a vacuum'tube includ f ing at least an anode, aflcathode, andga control'grid; an input circuit for saidsta'gecon necte-d tos'aid control grid; an" output con-3'7 plingcoil connected'betweenlsaid anode and. I said cathode ,a tuned circuit including se rie s a second "coil" coupled ingrevers'e sense to said "output coil, a tuning oondenser,-and a composite impedance including". a capacity magnetic coupling with saidsecon-d' coil and coil andsaid composite impedance, the inductance and capacity'of said-compositeim 1 pledan'ce being so relatedthatconditions for a substantial reactiye balance atflon'e frequencyresults in a reactive unbalance 'at oth er quencles w thinthe' range of ja'd ustment I ire ofeaiditunedcircuitf I I 2. An electr cal amplifier stage includlng' a vacuum tube having. at least ananode, a

cathode, and a control grid; an inputcircuitassociated with [said control I gridyfa tuned output circuit including a 'tuningcondenser ha'ving'one terminal connected to saidcathode,-and two coils having negligible electromagnetic coupling with each other, one or said .coils beingcoupled to acoilconnected between said anode and cathocle,xand the other of said coilsbeing associated with a capacity to form a composite impedance; and a" capacltive impedance connected between combination, an electron tube including at least an anode, a cathode, and a control means; an'input circuit associate'd with said control means; a C O1l associated with said anode and cathode; a second COlhCOllplQd to said first coil in the'reve'rse sense; a coin- I posite impedance including a capacity-and a third coil having negligibleelectromagnetic couplingwith said second coil, "said com: posite impedance having one terminal connected to said second coil; a tuning condenser connected between the-non-common terminals of said second coil and said composite impedance, a balancing circuit including capacitlve lmped'ance, sald balancing C11- cuit being connected between said control anda"th1rd'co1l havlng'negligible electro- I a'balancing condenserconnected between said 7 control grid and the unction of said second I reactive balance are variant with frequency, whereby adjustmentiof the stage to provide" 1 with slaid composite impedance; and a connec- I ance thereof of a type which increases with.

means and the. junctionv of said second coil,

tion between said cathode and the junctionof said compositeimpedance with said tun ng condenser;,saidelectrical circuitbeing so proportloned and arranged as to produce a controlled, progressive, retroactive unbalincrcasing wavelength "at. a predetermined rate over a. given range of frequencies. v

4. An electricalamplifier clrcult comprls I mg, in COIIIblIlltlOI'l, a Vacuum tube havmg at least an, anode, a cathode, a control. elementaan inputcircuit assoclated w1th: sa d 'controlelementand cathode; an outputcircuit including a .coil connected between, said anode anclcathode, azsecond coil coupled thereto lnthe reversesense, a composltelmpedan'ce connec'tedin series with said second coil and including a third coil having negligible electromagnetic coupling with, said second coillandfalcapacity, and a tuning condenser connected. across said secondcoil andsaid composite impedance 1n. series. to form a b tuned clrcu t; and a capacltive impedance.

arranged between said control element anda pomt. ln saldtuned clrcu t; saldelectrlcal =an1pl1fier c1rcu1t being so'proportloned and arranged asto produce a controlled, progress1ve,.retroact1ve unbalance thereof of a type which increases at'a predetermined. rate as the'tuned circuit is adjusted to resonance at frequencies further removed from the predetermined. frequency at'which the reactive a balance of the circuit is substantially per- I feet.

said unbalancingand said balance capacity a 5. An electrical amplifier circuit comprising, in combination, avacuum tube provided with anode, cathode. and control; electrodes, an output transformer having, its primary associated with said anode. and cathodeelements and its secondary eifectively reversed as regards its primary, acomposite imped ance .connectedto said secondaryv and including a balance coil having negligible electro magnetic coupling wlth said primary and.

secondary and an unbalanc'ing capacity, a tuning condenser-connected acrosspsaid secondary and composite impedance in series, and a balance capacity connected between said composite impedanceand said grid electrode,

being so proportioned that the circuit is nonoscillatory throughout the frequency range of said amplifier circuit when adjusted for reactive balance at the higher end of the frequency range.

Intestimony whereof, I aflix my signature.

LEWIS M- 

